Water flip-back

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The water flip-back scheme is usually incorporated in NMR pulse sequences to be applied on proteins to provide further sensitivity gain for labile NH groups avoiding saturation of the water magnetization ( 93JACS12593 , 94JMRA178-107 , and 95JMRB94-108 ). It has been usually incorporated in HSQC-like puse trains, during the zz-magnetization selection period, as a selective water excitation pulse followed by a defocusing gradient element.

Applications:

2D ¹H-¹⁵N HSQC experiment ( 93JACS12593 , 94JMRA178-107 , 95JMRB94-108 and 98JB279-11 ).

2D ROESY experiment ( 97JMR93-129 )

2D TOCSY experiment ( 95JMRB168-111 , 96JB213-8 , and 97JMR93-129 )

2D NOESY experiment ( 95JB327-5 and 97JMR93-129 )

3D ¹H-¹⁵N NOESY-HMQC experiment ( 98JMR87-135 )

Water-selective 90 pulse preceding the basic HMQC pulse sequence (¹H-¹⁵N FHMQC experiment). implementation in 3D NOESY-¹⁵-HMQC and 3D ¹⁵-HMQC-NOESY-¹⁵-HMQC experiments. ( 98JB279-11 )

2D ¹H-¹⁵N TROSY experiment ( 99JB77-13 ) in which the proton pulse phases are designed so that the water magnetization is flipped back to the +z axis before the WATERGATE scheme. Implementation in 3D ¹H-¹⁵N NOESY-TROSY and 3D ¹H-¹⁵N TOCSY-TROSY experiments.

As a general trend, a selective 270 self-refocusing gaussian-shaped pulse (applied on-resonance on the H₂O signal) around 2.5 ms is used for water flip-back purposes. The BRUKER's SYNTAX is

1H water flip-back pulse length: p11=2.5m

1H water flip-back pulse power (sp1): to be calibrated

1H water flip-back pulse shape: spnam1=gauss